Carbon isotopes in biology

Because of above-ground nuclear bomb testing, the neutrons released reacted with CO2 to increase the atmospheric 14C via the 14N (n, p) 14C reaction, which started rising, in about 1955 (Fig. 1) and reached a peak in the mid-1960s [50]. With the curtailment of above-ground nuclear testing in the 1960s, the atmospheric 14C concentration has since been decreasing exponentially (Fig. 1). This variation in 14C concentration can be used to establish when cells in biology were born and how quickly they are renewed [51]. This technique commonly is called carbon-14 bomb pulse biology, and it has provided information on the age of cells and their regeneration. Figure 2 shows the average age of selected cells in a 30-year-old human.

Stable isotope

Relative atomic mass

Mole fraction

12C 12 0.9893 13C 13.003 354 835 0.0107

Fig. 1: Global relative average atmospheric 14C concentration between 1950 and 2010 (modified from [50]).

Fig. 2: Average age of selected cells in a 30-year-old human, determined with 14C produced primarily in the 1960s as a result of above-ground, nuclear-weapons testing (data from [51-56]. This technique commonly is called carbon-14 bomb pulse biology. Occipital neurons in the cortex of the adult human brain are as old as the individual [51]. Lens crystallines are special proteins in the eye lens and are formed almost exclusively at birth with a very small, and decreasing, continuous formation throughout life [56]. Achilles tendon cells do not regenerate after the first 17 years of life [52]. Half of human fat cells are replaced about every 8 years [54].

Carbon isotopes in Earth/planetary science Because molecules, atoms, and ions of the stable isotopes of carbon possess slightly different physical, chemical, and biological properties, they commonly will be isotopically fractionated during physical, chemical, and biological processes, giving rise to variations in isotopic abundances and in atomic weights. Carbon in natural terrestrial materials shows a substantial variation in isotopic abundance (Figure 3), thus providing many different ways of distinguishing sources of materials and processes affecting them [1]. Variations in the isotope-amount ratio n(13C)/n(12C) in tree rings and of CO2 trapped in ice cores can be used to study causes of variations in atmospheric CO2 levels [57]. Variations in the isotope-amount ratio n(13C)/n(12C) and in the 14C concentration of surface ocean waters can be used to trace the incorporation and movement of atmospheric CO2 in the ocean [57].

Fig. 3: Variation in atomic weight with isotopic composition of selected carbon-bearing materials (modified from [1]).

Carbon isotopes in forensic science and anthropology

Variations in the isotope-amount ratio n(13C)/n(12C) of biological products can be observed using isotope-ratio mass spectrometry (IRMS) to detect adulteration (addition of inferior ingredients) in honey and other food products. The isotope-amount ratio n(13C)/n(12C) can fluctuate between carbon sources (C3 plants (grasses in temperate climates such as rice, potatoes, tomatoes and beet sugar), C4 plants (grasses in hot climates such as corn and sugar cane), animal carbon, atmospheric CO2, etc.). This commonly makes it possible to detect whether these different carbon sources have been mixed by using isotope or mass balance, for example to distinguish between beet sugar and cane sugar. The following adulterations commonly are detected using stable carbon isotope IRMS:

a. Variations in the isotope-amount ratio n(13C)/n(12C) of honey can be used to detect the addition (and potential adulteration) of high fructose corn syrup, corn, or sugar cane [58].

b. Variations in the isotope-amount ratio n(13C)/n(12C) of fruit juice can be used to detect the addition of a sugar [58].

c. Variations in the isotope-amount ratio n(13C)/n(12C) of natural vanilla extract can be used to detect the addition of artificial vanillin or p-hydroxybenzaldehyde [58].

d. Variations in the isotope-amount ratio n(13C)/n(12C) of beer can be used to detect C4 carbon, which would indicate that a beer company may have added ingredients that are not traditionally used in brewing beer. Therefore, this ratio can be used to detect the misrepresentation of a product as being pure [58, 59].

Stable carbon IRMS can be used to determine if the botanical origin of an alcoholic spirit has been mislabeled and if chaptalization (the process of adding sugar to increase the alcoholic content) of wine has occurred [58, 59]. 14C scintillation counting can be used to determine the age of wine and alcoholic spirits [58, 59]. Variations in the isotope-amount ratio n(13C)/n(12C) of urine has been used to determine if steroids in urine are natural or of synthetic origin. These measurements enable anti-doping laboratories to perfect their methods for detecting steroid doping in athletes [60-62]. Variations in the isotope-amount ratio n(13C)/n(12C) of marijuana can provide information to determine if the plants were grown “inside” a building or greenhouse or were “open grown” (Figure 4). Plant carbon isotopic compositions are controlled by atmospheric CO2 and the supply and demand of CO2 in photosynthesis (process used by plants to convert light energy from the sun into chemical energy). “Open grown” plants are grown in an area that is well ventilated and receives natural CO2; in contrast plants grown “inside” receive supplemented CO2 and the photosynthesis process is more confined. Additionally, CO2 from a tank of compressed gas used to augment atmospheric CO2 to increase growth of marijuana plants commonly is highly depleted in 13C as a refinery by-product. These differences change the carbon isotope ratios of the plants and the ratios vary enough to enable determination of the growing and cultivation process of marijuana [63, 64].

Fig. 4: Variations in the isotope-amount ratio n(13C)/n(12C) of marijuana can be used to determine if the plants were grown inside a building or greenhouse or were “open grown.” (Image Source: U.S. Drug Enforcement Administration and U.S. Department of Justice) [65].

Carbon isotopes in geochronology Radioactive 14C is the basis for the radiocarbon dating method to determine ages of carbon-bearing materials. 14C is formed naturally in the atmosphere by cosmic-ray interactions and it also was released by above-ground, nuclear weapons testing (Figure 1). Atmospheric 14C is incorporated into plants, animals, soils, groundwater, and ocean water, and it decays with a half-life of ~5700 years. This makes it useful for dating such things as archaeological remains and water masses in oceans and aquifers, on time scales ranging from hundreds of years to tens of thousands of years [3]. Plants and animals living since the 1950s can be identified by bomb-peak 14C in their cells. Carbon isotopes in medicine 14C can be used for isotopic labeling of new drugs to study their uptake and metabolism in humans [66-69]. 13C can be used in breath tests to detect Helicobacter pylori bacteria (bacteria in the stomach linked to ulcers), which can cause stomach ulcers and cancers [66, 67, 69-71].

Glossary atomic number (Z) – The number of protons in the nucleus of an atom. atomic weight (relative mean atomic mass) – the sum of the products of the relative atomic mass and the mole fraction of each stable and long-lived radioactive isotope of that element in the sample. The symbol of the atomic weight of element E is Ar(E), and the symbol of the atomic weight of an atom (isotope) of element E having mass number A is Ar(AE). Because relative atomic masses are scaled (expressed relative) to one-twelfth the mass of a carbon-12 atom, atomic weights are dimensionless. [return] electron – elementary particle of matter with a negative electric charge and a rest mass of about 9.109 × 10–31 kg. element (chemical element) – a species of atoms; all atoms with the same number of protons in the atomic nucleus. A pure chemical substance composed of atoms with the same number of protons in the atomic nucleus [703]. gamma rays (gamma radiation) – a stream of high-energy electromagnetic radiation given off by an atomic nucleus undergoing radioactive decay. The energies of gamma rays are higher than those of X-rays; thus, gamma rays have greater penetrating power. half-life (radioactive) – the time interval that it takes for the total number of atoms of any radioactive isotope to decay and leave only one-half of the original number of atoms. [return] isotope – one of two or more species of atoms of a given element (having the same number of protons in the nucleus) with different atomic masses (different number of neutrons in the nucleus). The atom can either be a stable isotope or a radioactive isotope. isotope ratio (R) – number (symbol N) of atoms of one isotope divided by the number of atoms of another isotope of the same chemical element in the same system [706]. [return] isotope-amount ratio (r) – amount (symbol n) of an isotope divided by the amount of another isotope of the chemical element in the same system [706]. [return] isotope-ratio mass spectrometry (IRMS) – the scientific field pertaining to use of mass spectrometer to measure the relative abundance of isotopes in a given sample, usually with an instrument having multiple ion collectors. [return] isotopic abundance (mole fraction or amount fraction) – the amount (symbol n) of a given isotope (atom) in a sample divided by the total amount of all stable and long-lived radioactive isotopes of the chemical element in the sample. [return] isotopic composition – number and abundance of the isotopes of a chemical element that are naturally occurring [706]. [return]

isotopic fractionation (stable-isotope fractionation) – preferential enrichment of one isotope of an element over another, owing to slight variations in their physical, chemical, or biological properties [706]. [return] isotopically labeled (compound) – a mixture of an isotopically unmodified compound with one or more analogous isotopically substituted compound(s) [703]. [return] metabolism – the chemical processes that occur within a living organism in order to maintain life [708]. [return] neutron – an elementary particle with no net charge and a rest mass of about 1.675 × 10–27 kg, slightly more than that of the proton. All atoms contain neutrons in their nucleus except for protium (1H). proton – an elementary particle having a rest mass of about 1.673 × 10–27 kg, slightly less than that of a neutron, and a positive electric charge equal and opposite to that of the electron. The number of protons in the nucleus of an atom is the atomic number. radioactive decay – the process by which unstable (or radioactive) isotopes lose energy by emitting alpha particles (helium nuclei), beta particles (positive or negative electrons), gamma radiation, neutrons or protons to reach a final stable energy state. radioactive isotope (radioisotope) – an atom for which radioactive decay has been experimentally measured (also see half-life). scintillation counting – measuring ionizing radiation using the interaction of radiation on a material and counting the resulting photon emissions. [return] stable isotope – an atom for which no radioactive decay has ever been experimentally measured. [return] X-rays – electromagnetic radiation with a wavelength ranging from 0.01 to 10 nanometers—shorter than those of UV rays and typically longer than those of gamma rays. References 1. M. W. Wieser, and Coplen, T.B. Pure Applied Chemistry. 83, 359 (2011). 3. I. D. Clark, and Fritz, P. Environmental Isotopes in Hydrogeology. Lewis Publishers, New York (1997). 50. M. B. Q. Hua, and A. Z. Rakowski. Radiocarbon. 55 (4), 2059 (2013). 51. R. D. B. K. L. Spalding, B. A. Buchholz, H. Druid, and J. Frisén. Cell. 122 (1), 133 (2005). 52. P. S. K. M. Heinemeier, J. Heinemeier, S. P. Magnusson, and M. Kjaer. The FASEB (Federation of American Societies for Experimental Biology) Journal. 27, 1 (2013). 53. a. E. J. v. S. G. D. Weinstein. Journal of Investigative Dermatology. 45 (4) (1965).

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